The present invention relates to high pressure sodium vapor high intensity discharge lamps, and more particularly to such lamps having a gas filled outer envelope.
High pressure sodium discharge lamps are comprised of a discharge device mounted in an evacuated outer envelope. The discharge device is typically a ceramic discharge vessel comprised of alumina or sapphire and having conductive terminals for receiving an operating voltage. The conductive terminals are niobium which is used because its coefficient of thermal expansion matches that of alumina and because it is resistant to sodium vapor. Titanium solder is used in connections to the niobium.
The outer envelope is evacuated in order to thermally isolate the discharge device, and to avoid reactions of any gas within the outer envelope with the discharge device. Nitrogen, which is used in the outer envelope of other types of high intensity discharge lamps, cannot be used in high pressure sodium lamps because of its reactivity with niobium and titanium at high temperature.
The evacuated outer envelope of high pressure sodium lamps must be strong and able to withstand severe mechanical impacts without breaking. If the lamp outer envelope were to break, it would implode scattering glass fragments and create a safety hazard.
It has been the practice to manufacture high pressure sodium lamps with evacuated outer envelopes, and to make those envelopes sufficiently strong to avoid breakage. However, high envelope strength is not feasible in the case of many reflector lamps. Reflector lamp envelopes have a large face that merges with the envelope side walls at an edge portion having a small radius of curvature. The atmospheric pressure acting on the evacuated envelope causes high stress concentrations in the edge portion and makes it susceptible to breakage. Moreover, reflector lamps have thin blown glass envelopes and cannot be strengthened by making them substantially thicker. Incandescent reflector lamps having blown glass envelopes uniformly contain a fill gas with an internal pressure of about one atmosphere. With the inner and outer pressures acting on the envelope being approximately equal, no implosion will occur if the envelope breaks and there is less apt to be flying glass fragments.
There has been some consideration of gas filled high pressure sodium lamps. U.S. Pat. No. 3,932,781 issued to Jozef C. I. Peeters et al. discloses a high pressure sodium lamp having an outer envelope that is gas filled to inhibit evaporation of the alumina discharge tube. This reduces the deposition of alumina on the outer envelope and the attendant reduction in light output. The results of experiments involving such a lamp are also disclosed in the article by R. J. Campbell et al., "Evaporation studies of the sintered aluminum oxide discharge tubes used in high pressure sodium (HPS) lamps", Journal of the IES, July 1980, pages 233-239.
The introduction of a fill gas into the outer envelope of a high pressure sodium discharge lamp presents the problem of voltage breakdown through the gas. These lamps have closely spaced metal parts having a potential difference of around 4000 volts during lamp operation. In the high vacuum of conventional high pressure sodium lamps electrical breakdown between the lamp parts was not a problem. A fill gas has the potential of ionizing and providing a conductive path between the internal lamp parts at the different potentials and electrical breakdown can occur.
Accordingly, it is an object of the invention to provide a high pressure sodium discharge lamp having a gas filled outer envelope in which electrical breakdown through the fill gas is prevented.